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Discussion Starter · #1 ·
I noticed my Xtrail seemed to be going very fast when looking at the speedo, yet didnt feel that fast. I hooked up my tomtom which I know to be pretty accurate from the other vehicles I use it in, and it was showing the Xtrail speedo is 5mph fast.
I know some cars do this as a kind of safety function, how does everyone elses compare?
Tyres are 215/60/17, which I believe is the correct size for this model, so is it normal for it to be 5mph fast?
 

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Hi

I reckon mine's not far out.

I was caught speeding on the M6 near Penrith by a safety camera on a motorway bridge. As we came round a bend on the motorway my wife spotted the van and a quick glance I read 81 mph on the speedo. I was done for exceeding the speed limit at 79 mph.

Personally I'm a bit suspicious of the accuracy of sat nav claims.

Xacre
 

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Discussion Starter · #3 ·
Usually I'd be suspicious of the sat nav too, but I use it in 4 different vehicles, 2 HGV's, one 3.5 ton van and another car and its always in sync with theose vehicles speedos.
 

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The same here drive a hgv and the tomtom is spot on in the wagon which is set to max of 56mph and shows the same reading but is 5 to 6 mph out in the xtrail ...mark
 

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I hadn't used my TomTom to check the speedo accuracy until the other week. I got the satnav to read 72 and i realised the speedo was showing 80!!!

No wonder folk were unhappy with me sitting at the speedos '70' when in reality I was actually travelling at 62!!!! :oops: :roll:
 

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You can calibrate the speedo on a quiet motorway as the markers are a fixed distance appart. I will have to dig out the details. you need to be able to drive a steady speed for at least a mile and get your passenger to time the interval. I last did this some time ago and I have forgotten the details. The markers are short (about 18" high) at I think 100m appart so take the timings at say 60 mph and do the sums at home.
The more miles you can maintain the speed the more accurate the result. I live near the M45 (Warwickshire) which is almost always deserted so I must have a go at it in cruise control.

Look at item 14 from the highways agency
http://www.highways.gov.uk/business/16049.htm
 

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Check my sums. 16 and a bit 100m markers (16.09 actually) so 16 should be near enough, = 1 mile.
speed is m/h,
so Actual Speed = 60x60/seconds to do the mile.

Taking 16 markers should be near enough, if you do the sums for a 60mph test the true corrected speed would be 59.664 (i.e. mph result for 16 markers x16/16.09 )
 

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Or you could just find a nice police car go past him doing 100mph and they will give you the exact speed on the ticket..................ha ha :thumbs:
 
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My speedo is 6mph fast at 70mph with respect to my Tom Tom. i.e 76mph on speedo is 70mph actual.
May I commiserate with the member who got caught for 79mph on the M6 near Penrith by a 'Cumbria Safety Camera' however they will do you for 78! - Source, Penrith magistrates! Be warned - don't speed in Cumbria. They have 4 mobile speed camera vehicles and are VERY active esp M6, A66, A686.
 

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Satnavs will always show a difference of around 5mph because of the time it takes the signal to bounce to and from the satellite. Unless you fit military spec satnav, you won't get better accuracy. Trust your speedo, or test it against Gatso :biggrin:
 

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Sat-navs are almost ALWAYS more accurate than the car speedo. By law, a mass-produced car speedo is allowed to OVER-read by 10%+2.5MPH more than the true speed, but it must NOT under-read under ANY circumstances. As the vehicles are mass-produced, they need to build some tolerance into the speedo calibration to allow for natural small variations between vehicles, so they always over-read by a bit. Different brands of tyre (even of the same size) will have a slightly different rolling radius, so they need to cater for the possibility of someone fitting 4 brand new ones of a bigger diameter and running them at maximum pressure. Finally, in vehicles where more than one size of tyre could be fitted, manufacturers will sometimes save money by keeping the same speed calibration in both - so one might over-read by a bit more than the other but both would still be within the law.

From personal experience, 5MPH at 70 is a fair bit for an excess reading. Most modern cars that I've had anything to do with are more like 2-3MPH over the odds at that speed.

Only thing I would say about sat-navs is that you need to be going in a straight line at roughly constant speed for several seconds to get the best out of them, because their own reaidngs will vary slightly every second or so.

Oh, and if it helps, I too know someone who got done for 79 on a clear, dry, and near-empty M6 in Cumbria too! They really are quite zealous (with very little to show for it in terms of accident reduction!) but this might cheer you up!

http://www.newsandstar.co.uk/news/redun ... rPath=news
 

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I used to own a Citroen Xsara Picasso. They have a digital speedo.
I owned two portable Satnavs a Garmin Streetpilot and a Tom Tom built into my phone. The speed on those always agreed to within a mph of my X-Pic (up till about 45-50 anyway). We also have locally some roadside speed sign's instead of Gatso's, they flash up your speed. Always within a mph of the X-Pic. So I can but conclude the x-Pic has a particularly accurate speedo. Plus they are a common sight on the road.

Soooooo when your passing an X-Pic have your Eagle eyed passenger look in the Top Centre of the dash to see the speed. You used to see people doing this all the time. Sorted.
:thumbs:
 

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knowing my luck instead of getting picked up for speeding i would also get picked up for trying to see into the another car... :biggrin:
 

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Re Speedo Accuaracy. By Legislation the speedo needs to be accurate +/- 10% when travelling in excess of 10mph. I am a retired Traffic Police Officer and from experience most vehicle manufacturer's speedo's are almost without fail nearly the full 10% fast (makes their vehicle look faster) so at 70mph your speedo will be showing somewhere around 77mph or therabouts.

BMcL
 

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Driving at 70mph (speedo reading) my Garmin shows 67mph. Driving at 50mph (speedo) the Garmin shows 47mph. Driving at 30mph Garmin shows 30mph. Not sure what this equates to (if anything) but thought I'd mention it.
 

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bmcl26 said:
Vandy
Looks like you seem to have a very accurate Speedo!!!

BMcL
Either that or the Garmin is out! Can't understand why the 30mph reading is the same but the faster reading are out?
I go by the car speedo as I know it keeps me within the limit. had a useful gizmo on my Merc - you could set a maximum speed and when you reached it, the throttle stopped at that position. It had cruise control but the speed limiter was handy as it didn't "fix" your speed and very handy when going through 50mph areas on the motorway - not always easy to keep to this.
 
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